Inhibition of AChE by malathion and some structurally similar compounds

Inhibition of bovine erythrocyte acetylcholinesterase (free and immobilized on controlled pore glass) by separate and simultaneous exposure to malathion and malathion transformation products which are generally formed during storage or through natural or photochemical degradation was investigated. Increasing concentrations of malathion, its oxidation product malaoxon, and its isomerisation product isomalathion inhibited free and immobilized AChE in a concentration-dependent manner. K-I, the dissociation constant for the initial reversible enzyme inhibitor-complex, and k(3), the first order rate constant for the conversion of the reversible complex into the irreversibly inhibited enzyme, were determined from the progressive development of inhibition produced by reaction of native AChE with malathion, malaoxon and isomalathion. KI values of 1.3 x 10(-4) M-1, 5.6 x 10(-6) M-1 and 7.2 x 10(-6) M-1 were obtained for malathion, malaoxon and isomalathion, respectively. The IC50 values for free/immobilized AChE, (3.7 +/- 0.2)10(-4) M/(1.6 +/- 0.1)10(-4), (2.4 +/- 0.3)10(-6)/(3.4 +/- 0.1)10(-6) M and (3.2 +/- 0.3)10(-6) M/(2.7 +/- 0.2)10(-6) M, were obtained from the inhibition curves induced by malathion, malaoxon and isomalathion, respectively. However, the products formed due to photoinduced degradation, phosphorodithioic O,O,S-trimethyl ester and O,O-dimethyl thiophosphate, did not noticeably affect enzymatic activity, while diethyl maleate inhibited AChE activity at concentrations GT 10 mM. Inhibition of acetylcholinesterase increased with the time of exposure to malathion and its inhibiting by-products within the interval from 0 to 5 minutes. Through simultaneous exposure of the enzyme to malaoxon and isomalathion, an additive effect was achieved for lower concentrations of the inhibitors (in the presence of malaoxon/isomalathion at concentrations 2 x 10(-7) M/2 x 10(-7) M, 2 x 10(-7) M/3 x 10(-7) M and 2 x 10(-7) M/4.5 x 10(-7)M), while an antagonistic effect was obtained for all higher concentrations of inhibitors. The presence of a non-inhibitory degradation product (phosphorodithioic O,O,S-trimethyl ester) did not affect the inhibition efficiencies of the malathion by-products, malaoxon and isomalathion

Interaction of some pd(II) complexes with Na+/K+-ATPase: Inhibition, kinetics, prevention and recovery

The aim of this work was to investigate the influence of [PdCl4](2-), [PdCl(dien)](+) and [PdCl(Me(4)dien)](+) complexes on Na+/K+-ATPase activity. The dose-dependent inhibition curves were obtained in all cases. IC50 values determined by Hill analysis were 2.25 x 10(-5) M, 1.21 x 10(-4) Mand 2.36 x 10(-4) M, respectively. Na+/K+-ATPase exhibited typical Michelis-Menten kinetics in the presence of Pd(II) complexes. Kinetic parameters (V-max, K-m) derived using Eadie - Hofstee transformation indicated a noncompetitive type of Na+/K+-ATPase inhibition. The inhibitor constants (K-i) were determined from Dixon plots. The order of complex affinity for binding with Na+/K+-ATPase, deducted from K-i values, was [PdCl4](2-) GT [PdCl(dien)]+ GT [ PdCl(Me(4)dien)](+). The results indicated that the potency of Pd(II) complexes to inhibit Na+/K+-ATPase activity depended strongly on ligands of the related compound. Furthermore, the ability of SH-donor ligands, L-cysteine and glutathione, to prevent and recover the Pd(II) complexes-induced inhibition of Na+/K+-ATPase was examined. The addition of 1mM L-cysteine or glutathione to the reaction mixture before exposure to Pd(II) complexes prevented the inhibition by increasing the IC50 values by one order of magnitude. Moreover, the inhibited enzymatic activity was recovered by addition of SH-donor ligands in a concentration-dependent manner

The influence of transition and heavy metal ions on ATP-ases activity in rat synaptic plasma membranes

The influence of transition metal (Cu2+, Zn2+, Fe2+ and Co2+) and heavy metal ions (Hg2+, Pb2+ and Cd2+) on the activities of Na+/K+-ATPase and Mg2+-ATPase isolated from rat synaptic plasma membranes (SPM) was investigated. The aim of the study was to elucidate the inhibition of both ATPase activities by exposure to the considered metal ions as a function of their affinity to bind to the SH containing ligand L-cysteine, as a model system. The half-maximum inhibitory activities (IC50) of the enzymes were determined as parameters of rectangular hyperbolas and correlated with the stability constant (Ks) of the respective metal-ion-L-cysteine complex. The linear Dixon plots indicate equilibrium binding of the investigated ions to both enzymes

The influence of potassium ion (K+) on digoxin-induced inhibition of porcine cerebral cortex Na+/K+-ATPase

The in vitro influence of potassium ion modulations, in the concentration range 2mM - 500mM, on digoxin-induced inhibition of porcine cerebral cortex Na+/K+-ATPase activity was studied. The response of enzymatic activity in the presence of various K+ concentrations to digoxin was biphasic, thereby, indicating the existence of two Na+/K+-ATPase isoforms, differing in the affinity towards the tested drug. Both isoforms showed higher sensitivity to digoxin in the presence of K+ ions below 20mM in the medium assay. The IC50 values for high/low isoforms 2.77 x 10(-6) M/8.56 x 10(-5) Mand 7.06 x 10(-7) M/1.87 x 10(-5) Mwere obtained in the presence of optimal (20mM) and 2mMK(+), respectively. However, preincubation in the presence of elevated K+ concentration (50 - 500mM) in the medium assay prior to Na+/K+-ATPase exposure to digoxin did not prevent the inhibition, i.e. IC50 values for both isoforms was the same as in the presence of the optimal K+ concentration. On the contrary, addition of 200mMK(+) into the medium assay after 10 minutes exposure of Na+/K+-ATPase to digoxin, showed a time-dependent recovery effect on the inhibited enzymatic activity. Kinetic analysis showed that digoxin inhibited Na+/K+-ATPase by reducing maximum enzymatic velocity (V-max) and K-m, implying an uncompetitive mode of interaction